How to Build the Perfect AOD - AODetail

Get performance and fuel economy with Ford’s solid and dependable Automatic Overdrive

Jim Smart

July 1, 2014

Photos By:
The Manufacturers,
George Reid

Ford’s hardy cable-modulated Automatic Overdrive (AOD) was produced from 1980 to 1991, then became an electronically controlled transmission known as the AOD-E in 1991. Later, the AOD-E became the 4R70W.

In the beginning, the AOD was your basic automatic transmission with a lock-up Overdrive feature, first available in the Mustang in 1984. The AOD was not intended to be used as a performance transmission, as evidenced by Ford’s use in everything from vans to Lincoln Town Cars.

Though the AOD was a new transmission in 1980, it was basically the FMX geartrain package in a fresh aluminum case with an Overdrive/lock-up unit incorporated into the existing gear-train. The FMX’s time-proven Ravigneaux planetary gear set, known for extreme durability, is the heart of all AOD, AOD-E, and 4R70W transmissions. The AOD’s weakest link is its 1.50-inch-wide Overdrive band and Reverse clutch drum, which didn’t improve until the new ’93 Lincoln Mark VIII, which got the wider 2.00-inch Overdrive band and Reverse clutch drum.

The other sticking point with the AOD is that pesky throttle-valve (TV) cable and the split-torque feature that annoys a lot of enthusiasts. The TV cable must be adjusted spot on with a pressure gauge along with a test drive and good gut shift feel in the seat of your pants or you risk burning up the AOD. The AOD’s TV cable, tied to throttle movement, determines line pressure at various throttle positions. The more cable tension (throttle opening), the more line pressure there is for firmer shifts. In other words, at open or wide-open throttle, we want greater line pressure at the clutches and bands for a firm shift without slippage. By the same token, at light or no throttle, we want a soft shift or downshift.

The TV cable does the work of both the vacuum modulator and kickdown linkage on C4, C6, and FMX transmissions. It eliminates the vacuum modulator and the kickdown linkage, which were often problematic. TV cable tension coupled with output shaft speed (governor) determines shift firmness and timing.

In your search for an AOD core, you will want a late ’80s production unit, though nearly all AOD units dating back to 1980 can be upgraded using the right internal parts. AOD main cases are all basically the same. It was the internal gear-train that improved over time. Early AODs had teething problems easily remedied with revised gear-train and valvebody parts. There are cast drums (early) and steel drums (later), which are lighter and stronger.

Mike Stewart at Mike’s Transmission in Southern California builds hundreds of AOD transmissions every year for customers around the world. He views the AOD, AOD-E, and 4R70W as hardy transmissions that could use improvement. He strongly suggests the wider AOD-E/4R70W Reverse drum and Overdrive band, which take the load better. He also suggests the “A” Overdrive servo for greater Overdrive band clamping pressure. And finally, he suggests the 4340 chrome-moly input shaft, which eliminates the stock input shaft breakage issues. Mike says that with the right parts, the AOD can take up to 800 hp. Some can be built to withstand 1,200 hp using aftermarket parts.

If you take a bare AOD case and fill it with the AOD-E or 4R70W geartrain, coupled with the 4340 input shaft, you will have a fiercely rugged and dependable AOD. The difference between the AOD-E and 4R70W is gear ratio. The 4R70W offers better gearing, which means hot acceleration for your small- or big-block Ford. The 4R70W is an improved AOD-E designed specifically for the 4.6L Modular V-8, which lacks the snappy low-end torque of a small- or big-block.

What bugs people most about the AOD are its two input shafts and that dysfunctional split-torque 60/40 function. The smaller removable secondary input shaft is tied directly to the torque converter’s shell and forward clutch. The larger hollow primary input shaft is driven by the torque converter’s turbine (torque multiplication) in First, Second, and Reverse gears.

Ford calls this split torquebecause 40 percent of the engine’s torque goes through the torque converter as torque multiplication (via the impeller, stator, and turbine), while 60 percent goes through the smaller input shaft in Third gear. When the transmission shifts into Overdrive or Fourth gear, 100-percent of the engine’s torque goes through the smaller secondary input shaft as straight drive and lock-up in the gear-train and clutch pack. Tip in the throttle and the AOD goes into Third gear with the split-torque pattern. At WOT, 100- percent of the torque goes through the torque converter and hollow primary input shaft.

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In your search for an AOD core, you will find a variety of AOD types from different applications, including the AOD-E and 4R70W. At a glance, they all look the same. Closer inspection reveals huge differences in the AOD, AOD-E, and 4R70W.

All of these transmissions are of the AOD family, but they are different. The AOD is a mechanically-modulated transmission with a throttle-valve cable designed to modulate line pressure, shift timing, and shift firmness. The AOD-E and 4R70W are computer-controlled transmissions void of the TV cable that make engine and transmission function cohesive. With this, function happens via multiplex plugs and the PCM (Powertrain Control Module).

There are aftermarket electronic shift control solutions for the AOD-E and 4R70W, so don’t count them out for vintage overdrive swaps either.

There are two ways to adjust the TV cable—with a pressure gauge or without. It is strongly suggested you use a pressure gauge at the line pressure port on the righthand side (passenger) of the transmission case and follow procedure to the letter. With a pressure gauge screwed into the line pressure port (a gauge with a length of hose makes the job easier), you should see 0-5 psi at idle speed. Under normal acceleration, you should see 30 psi. At wide open throttle, you should see 85 psi of line pressure. At idle, you should have slack tension on the cable, meaning no tension, but no slack either.

If you are not using a pressure gauge, cable tension should be slack, yet not slack. When you place the shifter in gear, there should be gentle engagement, not a jolt. Begin your test drive with light acceleration, which should deliver a firm upshift time with speed increase. Under hard acceleration, shifts should be delayed and quite firm. If there is slippage, cable tension must be increased. There can be no slippage under any circumstances. If there is, you’ll burn up the transmission.

During deceleration, you should not be able to feel the downshifts. If you can feel the jerky downshifts, there is too much cable tension and you have too much line pressure. Proper cable tension is about achieving a firm upshift during acceleration, void of any slippage.

What else are you going to do in Minnesota in the dead of winter but create terrific transmission adaptor kits for a wide variety of applications? Bendtsen’s Speed Gems Transmission Adaptors in Ham Lake, Minnesota, offers a variety of AOD transmission adaptors for Ford applications, including:

These kits enable you to install an AOD in nearly any vintage Mustang application with the exception of 170ci and 200ci inline-sixes.

AOD Conversion Kits

There was once a day when you had to shop piecemeal for all of the items necessary to swap Ford’s AOD into your vintage Mustang. Today, you can find nearly everything you’re going to need in one stop including the transmission. CJ Pony Parts has complete AOD conversion kits, including the transmission, for vintage Mustangs for under $3,500 that include everything you’re going to need to get on the road with an efficient overdrive.

For under $700, California Mustang can supply a conversion kit sans transmission. The beauty of California Mustang’s kit is a tubular crossmember that will clear just about every exhaust system imaginable. Included in the kit are the crossmember, 164-tooth flexplate, trans mount, linkage, TV cable, slip yoke, and installation hardware.

What bites AOD installers more than anything is improper torque converter installation. The AOD’s torque converter mandates extraordinary care and attention during installation to ensure you have seated the converter on the primary and secondary shafts, plus the stator support and front pump rotor. There should be three moments where the converter pops to the next position. The converter is properly seated when you can’t get your hand between the converter and bellhousing.

Failure to seat the converter means front pump damage and failure. If you start the engine and hear a squealing sound, the converter is not seated properly. Shut down the engine immediately.